Libert was a pre-dreadnought battleship built for the French Navy in the mid-1900s. She was the lead ship of the Libert class, which included three other vessels and was a derivative of the preceding Rpublique class, with the primary difference being the inclusion of a heavier secondary battery. Libert carried a main battery of four 305-millimeter (12 in) guns, like the Rpublique, but mounted ten 194 mm (7.6 in) guns for her secondary armament in place of the 164 mm (6.5 in) guns of the earlier vessels. Like many late pre-dreadnought designs, Libert was completed after the revolutionary British battleship HMS Dreadnought had entered service, rendering her obsolescent.

On entering service, Libert was assigned to the 2nd Division of the Mediterranean Squadron, based in Toulon. She immediately began the normal peacetime training routine of squadron and fleet maneuvers and cruises to various ports in the Mediterranean. She also participated in several naval reviews for a number of French and foreign dignitaries. In September 1909, the ships of the 2nd Division crossed the Atlantic to the United States to represent France at the HudsonFulton Celebration.

Libert's active career was cut short on 25 September 1911 when a fire broke out in one of the ship's propellant magazines and led to a detonation of the charges stored there, destroying the ship in a tremendous explosion that killed 286 of her crew. The blast also damaged several other vessels and killed crewmen on six neighboring ships. An investigation revealed that the standard French propellant, Poudre B, was prone to decomposition that rendered it very unstable; it had likely been the culprit in several other ammunition fires in other ships. The wreck remained in Toulon until 1925, when her destroyed hull was refloated, towed into a drydock, and broken up.

A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or other motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the engine that expels the propellant is called a reaction engine. Although technically a propellant is the reaction mass used to create thrust, the term "propellant" is often used to describe a substance which is contains both the reaction mass and the fuel that holds the energy used to accelerate the reaction mass. For example, the term "propellant" is often used in chemical rocket design to describe a combined fuel/propellant, although the propellants should not be confused with the fuel that is used by an engine to produce the energy that expels the propellant. Even though the byproducts of substances used as fuel are also often used as a reaction mass to create the thrust, such as with a chemical rocket engine, propellant and fuel are two distinct concepts.

Vehicles can use propellants to move by ejecting a propellant backwards which creates an opposite force that moves the vehicle forward. Projectiles can use propellants that are expanding gases which provide the motive force to set the projectile in motion. Aerosol cans use propellants which are fluids that are compressed so that when the propellant is allowed to escape by releasing a valve, the energy stored by the compression moves the propellant out of the can and that propellant forces the aerosol payload out along with the propellant. Compressed fluid may also be used as a simple vehicle propellant, with the potential energy that is stored in the compressed fluid used to expel the fluid as the propellant. The energy stored in the fluid was added to the system when the fluid was compressed, such as compressed air. The energy applied to the pump or thermal system that is used to compress the air is stored until it is released by allowing the propellant to escape. Compressed fluid may also be used only as energy storage along with some other substance as the propellant, such as with a water rocket, where the energy stored in the compressed air is the fuel and the water is the propellant.

In electrically powered spacecraft, electricity is used to accelerate the propellant. An electrostatic force may be used to expel positive ions, or the Lorentz force may be used to expel negative ions and electrons as the propellant. Electothermal engines use the electromagnetic force to heat low molecular weight gases (e.g. hydrogen, helium, ammonia) into a plasma and expel the plasma as propellant. In the case of a resistojet rocket engine, the compressed propellant is simply heated using resistive heating as it is expelled to create more thrust.

In chemical rockets and aircraft, fuels are used to produce an energetic gas that can be directed through a nozzle, thereby producing thrust. In rockets, the burning of rocket fuel produces an exhaust, and the exhausted material is usually expelled as a propellant under pressure through a nozzle. The exhaust material may be a gas, liquid, plasma, or a solid. In powered aircraft without propellers such as jets, the propellant is usually the product of the burning of fuel with atmospheric oxygen so that the resulting propellant product has more mass than the fuel carried on the vehicle.

Proposed photon rockets would use the relativistic momentum of photons to create thrust. Even though photons do not have mass, they can still act as a propellant because they move at relativistic speed, i.e., the speed of light. In this case Newton's third Law of Motion is inadequate to model the physics involved and relativistic physics must be used.

In chemical rockets, chemical reactions are used to produce energy which creates movement of a fluid which is used to expel the products of that chemical reaction (and sometimes other substances) as propellants. For example, in a simple hydrogen/oxygen engine, hydrogen is burned (oxidized) to create H2O and the energy from the chemical reaction is used to expel the water (steam) to provide thrust. Often in chemical rocket engines, a higher molecular mass substance is included in the fuel to provide more reaction mass.

Rocket propellant may be expelled through an expansion nozzle as a cold gas, that is, without energetic mixing and combustion, to provide small changes in velocity to spacecraft by the use of cold gas thrusters, usually as maneuvering thrusters.

To attain a useful density for storage, most propellants are stored as either a solid or a liquid.